Datasheet

 1996 Microchip Technology Inc.
Preliminary
DS40122B-page 61
PIC14000
8.6 Pr
ogrammable Current Source
Four configuration bits (ADCON1<7:4>) are used to
control a programmable current source for generating
the ramp voltage to the A/D comparator. It allows com-
pensation for full-scale input voltage, clock frequency
and CDAC capacitor tolerance variations. The current
values range from 0 to 33.75 µ A (nominal) in 2.25 µ A
increments. The intermediate values of the current
source are as follows:
TABLE 8-2: PROGRAMMABLE CURRENT
SOURCE SELECTION
ADCON1<7:4>
Current Source
Output
0000OFF - all current
sources disabled
00012.25 µ A
00104.5 µ A
00116.75 µ A
01009 µ A
010111.25 µ A
011013.5 µ A
011115.75 µ A
100018 µ A
100120.25 µ A
101022.5 µ A
101124.75 µ A
110027 µ A
110129.25 µ A
111031.5 µ A
111133.75 µ A
The programmable current source output is tied to the
CDAC pin and is used to charge an external capacitor
to generate the ramp voltage for the A/D comparator.
(Refer to Figure 8-1.) This capacitor should have a low
voltage-coefficient as found in teflon, polypropylene, or
polystyrene capacitors, for optimum results. The
capacitor must be discharged at the beginning of each
conversion cycle by asserting ADRST (ADCON0<1>)
for at least 200 µ s to allow a complete discharge.
Asserting ADRST disables the current sources inter-
nally. Current flow begins when ADRST is cleared.